The laws regarding using lethal force against an intruder are generally lenient here because of an understanding that someone who's gone so far as to invade your home may not have very good intentions for your well-being. Same with being threatened with a weapon: if someone has a gun drawn on you and you immediately pull your own weapon and pop him in the head before he can shoot, it'll likely be considered justifiable self-defense.
It becomes a different story depending on what happens during the fight, however. You can't attack someone who's retreating or surrendering, and you MUST stop when the person has been disabled. This doesn't mean not to dump your entire magazine into an attacker, but you stop shooting or stabbing or punching when he falls. If the guy's on the ground unconscious and you stomp on his head, that's battery. Shoot him, and you get a murder charge.
Another aspect is whether or not the attacker has a weapon. Again, there's more lenience if the guy is intruding in your home. But there's always a "match force for force" rule. If the person is unarmed, you can't shoot unless you reasonably believe that he's about to commit murder with his bare hands. I personally held a rifle on someone who was assaulting his girlfriend and their infant daughter, and it may have been justifiable to shoot to save the baby if he didn't stop when ordered because of her fragility. Though that's a very gray area. If the guy has a weapon, though, all bets are off: the moment you brandish a weapon or begin to draw one, you're free game to kill.
So let's say someone was to walk into my house through an unlocked door right now. If I gave him one warning while holding a weapon on him, he'd basically have no choice but to flee immediately. If he made any attempt to attack me, I could stab or shoot or bludgeon him in self-defense, but I'd need to stop as soon as he dropped. If he drew a gun on the ground or got back up and attacked, I could attack again.
Unfortunately, there's very little info on this case and there probably won't be any specifics for a long time. The investigators will need to examine the scene and the testimony of everyone involved (and alive) to try and piece together what happened.